Heretofore much effort has been made to realize lightweight and low-cost concreted products utilizing as filler material lignocellulosic fragments, and employing as bonding agent some form of mineral cement, to produce building materials. Typical cements have comprised Portland and other hydraulic cements and pozzolans, and magnesia cements such as Sorel cement. The problems of forming products of adequate strength and with densities less than unity arise because of inferior junction bond strength, that is, the adherence of the mineral mass to the woody filler. An understanding of the composition of wood fragments, and the chemistry of the reactants producing the mineral bond mass, may be gained by considering the following discussion.
Woody plant structures comprise arrays of hollow cells (tracheids) having fiber constituents comprised as cellulose which is present as high-polymer strong micro-fibril wall structure surrounded by non-fiber carbohydrate, constituents which encompass the lignins, sugars, starches, extractives, proteins polyphenolics, resins, waxes, fatty substances, and gums. Various sequestered minerals may also be occluded, chiefly silica. When woody plant materials are comminuted to product fragments by mechanical processes a wide range of fragment shapes, porosites, pore opening area, sizes and ratios of surface area to volume can be produced; for example, the fragments may be crumb-like sawgenerated particles and dusts; rough lumps, slivers and dusts made by hammer milling or hogging; pulp fibers made by wet grinding; and shavings, veneers, strands, wood-wools and excelsiors made by slicing with knife-edge tools.
The prior art has shown amply that although fragments of a range of sizes may be admixed with a minor volume proportion of cementitious binder material such as a hydraulic cement or magnesium oxychloride cement, the cast product may be useless due to poor bonding between the gelling bond mass and the fragment surfaces, or due to poor setting or even total absence of setting of the bonder remants. For example, any attempt to make a lightweight concreted product using Western Red Cedar (Thuja Plicata) fragments made from bark-free boles, combined with Portland cement, results in a non-setting mixture which never hardens or forms a bond. The failure to achieve set has been ascribed to the detrimental effects on the cement hydration process of certain extractives, chiefly wood sugars and polyphenolics. Poor sets with hydraulic cements also result from combinations of many other wood species, notably most tropical woods, and at the present time cement-bonded compositions are restricted to a few species such as the spruces (Picea Stitchensis), true firs (Abies Sp.), aspens (Populus Sp.) and some pines (Pinus palustris, Pinus Lambertiana). However, even preferred species which have lesser amounts of adversely-reacting extractables require precleaning and migration-blocking treatments to either remove inhibiting substances, or to seal fragment surfaces, or to convert near-surface contaminants to innocuous residues. The contaminating substances present in untreated fragments become partly dissolved as water from the cement slurry migrates through the fragment openings, and the extracted material has ample time to become distributed in the slurry to impede or prevent gelling; it may also be conjectured that the fragment surfaces become coated with extracted material, impairing the adherence of the bond mass.
Typical treatments hitherto resorted to comprise: (a) impregnating fragment surfaces with soluble metal salts such as chlorides of calcium or magnesium, which hasten the set of hydraulic cement slurry adjacent the fragment; (b) digesting extractables at and near fragment surfaces by treatment with baths of lime or caustic soda, with or without further stabilizing by a pozzolan or a polyvalent metal salt; and (c) loading surfaces of fragments with a mineral gel, e.g. sodium silicate. Despite such costly pre-treatment procedures, which necessitate at least an additional drying step, the adhesion of the mineral bond mass is relatively poor, as compared for example with that of thermosetting resinous adhesives currently employed for bonding wood fragments as boards. The inferior adhesion of such prior junction bond masses to wood fragments is thought to arise from the failure to develop a gel phase of reacting binder materials extending within pores and lumen openings presented at fragment surfaces, with consequent non-integral deposit of mineral bond mass following hardening. Such bonding as is observed is speculated to be mainly the result of embedment of fragment portions by a partial matrix of the bond mass. Examples of prior art lignocellulose/mineral composite products are described in United Kingdom specification No. 1,089,777 of Nov. 8, 1967, and in U.S. Pat. Nos. 2,175,568 of Oct. 10, 1939 to Haustein, 2,837,435 of June 3, 1958 to Miller et al, and 1,568,507 of Jan. 5, 1926 to Jaeger. Nailing concretes are described in United States Department of the Interior, Bureau of Reclamation text "Concrete Manual", Sept. 1949, pp. 351-352.
The prior art has proposed dry cement compositions comprising phosphorous containing compounds including phosphoric acid, and basic metal oxides such as aluminum and magnesium oxides and their oxyphosphate compounds, settable on mixing with water to form a concrete binder, as disclosed in U.S. Pat. No. 3,525,632 dated Aug. 25, 1970 issued to Enoch, C. R. Such cements are intended to be used with mineral aggregates.
A number of prior workers have combined water-soluble or dissolved acid phosphates with magnesia in rapid-setting compositions incorporating refractory fillers. In U.S. Pat. No. 2,456,138 issued to Wainer, dated Apr. 5, 1949, a mold of this type uses ammonium di-acid phosphate and dead-burned magnesia as cementing constituents which set rapidly to a refractory solid. Gunning mixes for repairs to linings of metal-melting furnaces have proposed alkaline polyphosphates, iron oxide, with magnesia or chromite non-acid constituents, in U.S. Pat. No. 3,278,320 dated Oct. 11, 1966, issued to Neely et al. Suggested polyphosphates named include sodium and ammonium polyphosphates.
Another wet-chemistry composition intended for a casting or pressing mix in repairing furnace linings has been proposed by Limes et al in U.S. Pat. No. 3,285,758 issued Nov. 15, 1966 using ammonium polyphosphates in water solutions of analysis 10% ammoniacal nitrogen and 34% P.sub.2 O.sub.5, further diluted and mixed with a minor weight proportion of magnesia or calcined magnesite and a major proportion of refractory aggregates.
A similar composition intended for very rapid chemical reaction and gunning application to hot oven walls, disclosed in U.S. Pat. No. 3,413,385 issued Nov. 26, 1968 to Komac et al, utilizes mixed ammonium phosphates combined in the gun with a minor weight proportion of magnesia dispersed in a dry aggregate.
It has been proposed in U.S. Pat. No. 3,821,006 issued June 28, 1974 to Schwartz to make a castable concrete by admixing water to dry constituents comprising dead-burned magnesia, an acid phosphate salt such as ammonium orthosphosphate, and a finely-divided inert aggregate, utilizing heat of the exothermic reaction yielding magnesium phosphate binder to set the concrete within minutes and develop a cure within a day.
A fast-setting concrete of low porosity disclosed in U.S. Pat. No. 3,879,209 issued Apr. 22, 1975 to Limes et al, is made with 15 parts by weight of 34 percent P.sub.2 O.sub.5 ammonium polyphosphate solution and an equal weight of -150 mesh magnesia, admixed with 70 parts limestone, dolomite, sand, or gravel, 3 parts salt, and 15 parts water; such concrete develops an early set and high compressive strength in a few hours.
The formulation of a substantially dry mix wherein one constituent is an acid that is normally liquid such as hydrochloric or phosphoric, or ammonium polyphosphate solution, and another constituent is alumina, dead-burned magnesia or chromite, is proposed in U.S. Pat. No. 3,475,188 dated Oct. 28, 1969 issued to Woodhouse et al, wherein the liquid chemical is adsorbed in a chemically inert pulverulent mineral solid such as bentonite or kieselguhr. Upon adding water to the dry mix the bond-forming liquid is flushed from the absorbent mineral to react with the base constituents to form a refractory solid.
While the formulations hitherto proposed are efficaceous in binding granular mineral aggregates such as refractories to form high-density products, the substitution of wood fragments in the mixes has failed to produce high-strength, low density structural products. Only when a major volume proportion of mineral cementing materials is present, yielding product densities well above 1.5 characterized by nearly complete occlusion of any fragment in a mineral matrix, can products of significant compressive strength be realised. However, the composite product proves weak in shear and tension, due to inferior bond adhesion.
The binding together of mineral solids by a cementing agent essentially requires that such agent be a viscous liquid, paste, or slurry, capable of wetting all surfaces of the mineral solids while plastic, and capable of gelation and development of interlocked crystal groups adhered to at least some portions of the aggregate materials. Cast bodies have high compressive strengths due to the effective support column created by the cement matrix surrounding the aggregate grains, but low tensile strength due to relatively low shear strengths of the bonds.
The problem of producing a truly strong lignocellulose/mineral composite product has not heretofore been met. As an objective, the production of low weight structural products with bulk density under 0.8, and preferably under 0.65, with Modulus of Rupture in static bending above 10 kg/cm.sup.2 and preferably above 100 kg/cm.sup.2, is highly desirable in order to provide cheap building materials utilizing forest and crop residues. Generally, the proportional limit of fiber stress for average woods in tension is above 350 kg/cm.sup.2 in air-dry condition; the weakest strength is in shear parallel to the grain, usually above 35 kg/cm.sup.2. It can therefore readily be appreciated that provided a sufficiently strong junction bond can be developed between assembled wood fragments, structural products potentially having excellent Modulus of Rupture strength properties would be feasible. The following discussion elucidates the problem of achieving such structures.